What is May Day, anyway?

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Indonesian workers in Jakarta celebrate May Day with a protest march, not a maypole.

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May Day falls on May 1 every year

It is a holiday of summer celebration, but also marks International Worker's Day

(CNN)To most people in the Northern Hemisphere, May Day conjures images of brightly colored twirling ribbons and promises of warm days ahead. That's not the whole story, though: May Day is also a day of protests and riots that traces its modern roots back to a world-changing explosion in Chicago.

When is May Day?

May Day is May 1 every year. Easy to remember, right?

What is May Day?

Depending on where you are, it's either a seasonal celebration or a day to celebrate workers' rights, or maybe a little bit of both. Think of the latter use as a Labor Day, if you will, for the rest of the world.

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How did it start?

This is a more complicated question. Originally, May Day was an ancient pagan holiday celebrating the start of summer. In Gaelic traditions, it is known as Beltaine (or the Anglicized "Beltane"). As time went on, different groups adapted the celebration to their specific cultures or beliefs. Europeans and Americans often celebrate in a more secular manner with diversions like maypole dancing and flower crowns. (That certainly lends a bit of cultural context to all the young women breezing around summer music festivals this time of year with giant daisies on their heads.)

Also of note: In May, the Southern Hemisphere is getting ready for winter, so May Day as a seasonal celebration is, for the most part, a Northern Hemisphere thing.

How did it become a day for labor rights?

May Day is also a labor holiday in many areas of the world, and that part of its history is a thornier story. May Day has shared a date with International Workers' Day since the 1880s. At the time, labor movements around the world were fighting for fair work accommodations like eight-hour workdays and unions. The date was chosen because it aligned with the anniversary of the Haymarket affair in Chicago, where police killed four people at a peaceful protest after someone threw a bomb into the crowd.

The event had a huge impact on labor movements across the world.

Photos:2015 May Day demonstrations

Photos:2015 May Day demonstrations

Police use water cannons on May Day demonstrators in Istanbul, Turkey, on Friday, May 1. Clashes erupted between police and protesters, who defied a government ban on marching to Taksim Square. Rallies around the world marked May Day, referred to as International Workers' Day in many countries.

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Photos:2015 May Day demonstrations

Labor union members join workers and activists in Jakarta, Indonesia. Protesters were demanding better working conditions.

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Photos:2015 May Day demonstrations

Contract workers and government employees shout slogans during a protest in Srinagar, India.

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Photos:2015 May Day demonstrations

Members of the Sri Lankan Marxist political party People's Liberation Front march during a rally in Colombo, Sri Lanka.

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Photos:2015 May Day demonstrations

Police detain a man at the May Day rally organized by the Communist Party of Ukraine in Kiev.

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Photos:2015 May Day demonstrations

France's far-right National Front leader Marine Le Pen, center, delivers a speech during the May Day march in Paris.

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Photos:2015 May Day demonstrations

Garment workers and activists shout during a May Day rally demanding a better work environment in Dhaka, Bangladesh.

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Photos:2015 May Day demonstrations

Members of Russian trade unions rally at Red Square in Moscow.

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Photos:2015 May Day demonstrations

Protesters hold up a banner during a May Day rally in Athens, Greece.

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Photos:2015 May Day demonstrations

Migrant workers gather in Hong Kong to call for better working conditions.

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So why are there sometimes riots and marches on May Day?

Because of its more recent history, International Workers' Day/May Day is often a day of protest for labor unions around the world. The people come come out to rally, and sometimes their passionate demonstrations can turn violent. In 2014, Turkey attempted to ban labor rallies, citing security concerns. Across Europe, similar events have attracted heavy police presence.

In a strange way, some of these demonstrations overlap with the more festive roots of May Day: The planned protests in Seattle include a rock concert, and the long-running May Day Parade in Minneapolis features both colorful, festive floats and revelers who wear satirical costumes related to the labor and political issues of the day.